The present invention relates to a method of and a system for controlling a hydraulic pressure, i.e., a line pressure, under which a hydraulic fluid is supplied from a pump driven by an engine toward a hydraulically actuable coupling of an automatic transmission drivingly coupled with the engine.
It is known to set the magnitude of line pressure to the maximum level when a temperature of the hydraulic fluid, i.e., a transmission oil temperature, is less than a predetermined temperature value, i.e., -10.degree. C., so as to cope with increased flow resistance owing to increased viscosity of hydraulic fluid at such low temperatures. Thus, under this condition, the magnitude of load imposed on the engine by the pump is substantially great, making smooth cranking and subsequent warming-up operation of the engine difficult.
An object of the present invention is to improve a hydraulic pressure control such that smooth cranking and subsequent warming-up operation of the engine is assured when the temperature of the hydraulic fluid is less than the predetermined temperature value.